In Rhino you can’t substract a volume from another volume that encloses it entirely.
For instance two spheres, smaller inside a bigger one. You can’t subtract the smaller from the larger.
Rhino can subtract only surfaces that intersect.
Run boolean Union on the tube and disks at A.
Run the command NonmanifoldMerge with the DeleteInput=Yes option and select the unioned object from 1.
Run the command CreateRegions and select the non-manifold object from 2.
Select the unwanted disk in the middle of the regioned object from 3 (don’t select the part you want to keep!) and delete it.
You should now have what you want.
It used to be that Rhino could not create an inner void unconnected to the outer surfaces: these relatively recent commands add that capability.
Right, kinda forgot about that. Still pretty hard to do though, IMO.
Thank you! [jeremy5] I’ve learned it.
CreateRegions ( I’ve found No tool bar & No Menu)